1. Field of the Invention
The present invention relates to a liquid crystal display device formed by plane parallel carrier plates each having associated electrodes and linear polarizers, wherein a twisted nematic liquid crystal is provided between the plates to form a cell, and a light source illuminates the cell.
2. Description of the Prior Art
From "Electronic Engineering", volume 46, no. 555 (May 1974), page 17, various types of illumination for liquid crystal displays are known. Initially, the direct and indirect illumination of displays in transmission operation are described. In order to obtain good illumination, the angle of incidence of the light relative to the perpendicular on the surface should be selected not greater than 45.degree., since otherwise the contrast becomes too small. An angle of incidence of 45.degree. is obtained by means of a louvre foil. In reflection operation, the display device is built into a recess and, evidently, a scattering layer is provided directly in front of the display device. A reflector layer is then fitted behind the display. A black plate is arranged obliquely above the display on the observer side, in order to absorb interfering scattered light. This arrangement is used, for example, in electronic computers.
From IBM Technical Disclosure Bulletin, volume 20, no. 113 of April 1978, a multiplexed liquid crystal display with a large viewing angle is known. It is operated in transmission. This display device comprises two polarisers and a thin scattering foil which is located on the observer side. The display is illuminated with quasi-parallel light incident obliquely. The angle of incidence 90.degree.-.delta. of the light relative to the perpendicular on the surface is about 20.degree..
The liquid crystal displays described, which are operated in transmission, have various disadvantages. Firstly, the legibility is considerably impaired if a display is viewed in the direction of the illumination source. Secondly, there is an interfering parallax; for this reason, the angle of incidence relative to the perpendicular on the surface cannot be selected to have any desired magnitude. Additionally, such devices have a multiplex rate which is not particularly large (up to about 1:30). The use of a scattering foil has the effect that only about half of the incident light is transmitted. As a result, the readability is considerably impaired.
The known reflective display has the disadvantage that it requires a fairly involved arrangement. In addition, the scattering layer considerably reduces the contrast and the black absorption plate restricts the visual range quite significantly. Such a display also has low multiplex rates.